The present invention is direction to a method for reducing the stimulated Brillouin backscatter or SBS in the transmission of light pulses or light waves via an optical fiber.
When, given transmission of optical signals within a specific bandwidth, the Brillouin bandwidth, the power of the signal supplied into the fiber exceeds a specific value referred to as a critical power or stimulated Brillouin scattering or SBS threshold, an optical backscatter is caused that leads to the fact that the transmitted power rises only sub-proportionally.
An article by R.G. Smith entitled xe2x80x9cOptical Power Handling Capacity of Low Loss Optical Fibers as Determined by Stimulated Raman and Brillouin Scatteringxe2x80x9d from Applied Optics. Vol. 11, No. 11, November 1972, pp. 2489-2494 describes these laws concerning scattering.
In an article by Roger Stolen entitled xe2x80x9cNonlinear Properties of Optical Fibersxe2x80x9d from Optical Fiber Telecommunications, Academic Press, 1979, pp. 125-150, the relationship between power and Brillouin backscatter for narrow and large bandwidths is described (see pages 133-134 of this article).
An article by Yamamoto et al entitled xe2x80x9cCoherent Optical Fiber Transmission Systemsxe2x80x9d from IEEE Journal of Ouantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 919-934, recites an equation on page 927 wherein the bandwidth of the signal source was replaced by the bandwidth of the modulated signal. The author points out on the following page that the critical power for the Brillouin backscatter is dependent on the signal spectrum and increases with increased data rate, which, as known, leads to a broader spectrum. As in the above-mentioned article by Stolen, the condition recited in Smith that only the power within the Brillouin bandwidth is considered, of course, must be taken into consideration in the indicated estimate or a corresponding effective bandwidth must be identified.
Cotter (U.S. Pat. No. 4,560,246) employs angled modulation, which, as known, increases the bandwidth in order to reduce the Brillouin backscatter.
It is also known from an article by Carman et al entitled xe2x80x9cTheory of Stokes Pulse Shapes in Transient Stimulated Raman Scattering*xe2x80x9d Physical Review A, Vol.2, No.1, July 1970, pp. 60-72, that chirping of pulses increases the bandwidth. As a result thereof the Brillouin backscatter is reduced according to the above-mentioned articles by Stolen and Yamamoto et al. This chirping creates a frequently undesired effect in the direct intensity modulation of a laser.
All of these effects and measures lead to a bandwidth of a light wave supplied into an optical fiber or of a supplied light pulse, which is always increased and which results in other disturbing effects.
The self-phase modulation that leads to an increase in the frequency at the beginning of a pulse and then to a lowering of the frequency also effects a spread of the spectrum and a reduction of the Brillouin backscatter as soon as the Brillouin bandwidth is exceeded. A considerable self-phase modulation that leads to a substantial SBS reduction can be generated by optical semiconductor amplifiers, wavelength converters and other non-linear components.
An article by Horiuchi et al entitled xe2x80x9cStimulated Brillouin Scattering Suppression Effects Induced by Cross-Phase Modulation in High Power WDM Repeaterless Transmissionxe2x80x9d from Electronics Letters, Vol. 34, No. 4, Feb. 19, 1998, pp. 390-391, investigates the effect of cross-phase modulation on the spectra of WDM channels, wherein WDM is wavelength-division multiplex. Given this effect, the various transmission channels influence one another so that a spread of the spectrum likewise occurs. However, a relevant SBS suppression only occurs at high powers.
The object of the present invention is to provide a method for reducing the stimulated Brillouin scattering, wherein an additional enlargement of the bandwidth of the infed optical signal is not required.
This object is achieved by the utilization of non-linear effects according to an improvement in a method for transmission of intensity modulated light waves or light pulses via an optical fiber, wherein the light pulses comprise a spectral power density distribution that, without additional measures, causes Brillouin backscatter, with the improvement being generating, in the fiber, a spread of the frequency spectrum that reduces the stimulated Brillouin backscatter by a self-phase modulation.
In addition, the invention is directed to an improvement in a method for parallel transmission of a plurality of intensity modulated light waveguides or light pulses having different wavelengths via an optical fiber, wherein the light waves and light wave pulses comprise a spectral power density distribution that, without additional measures, causes Brillouin backscatter. The improvements are influencing the light waves and light wave pulse sequences by a cross-phase modulation so that the frequency spectrum is enlarged in at least one of the waveguides so that the stimulated Brillouin backscatter is reduced.
First, the self-phase modulation is utilized, and this leads to an increase in the frequency of the beginning of the pulse and then to a lowering of the frequency. This means a spread of the spectrum that, as soon as the Brillouin bandwidth is exceeded, leads to a reduction of the Brillouin backscatter. What is the determining factor for this procedure is the Kerr effect, which is described in the applicable literature.
Another possibility is to utilize the cross-phase modulation between different light waves given wavelength-division multiplex systems. In this effect, the various transmission channels influence one another so that a spread of the spectrum likewise occurs. By optimizing the mutual influencing, for example by selecting suitable frequency spacings between the transmission channels, this effect can be optimized.
Another solution is achieved by an improvement in a method for suppressing the Brillouin backscatter in a transmission of the intensity-modulated optical transmission signal or a plurality of intensity-modulated optical transmission signals with different wavelengths over an optical fiber, whereby the optical signals comprise a spectral power density distribution that, without additional measures, causes stimulated Brillouin backscatter. The improvement is the supplying into the optical fiber of at least one amplitude-modulated pump signal that influences the optical transmission signals by cross-phase modulation so that the stimulated Brillouin backscatter is reduced.
The application of the invention given optical wave division multiplex transmission systems is especially advantageous. The stimulated Brillouin scattering or backscatter reduction already effected by the cross-phase modulation can be significantly improved by at least one amplitude-modulated pump signal. Given a standard monomode fiber, the infeed of at least one amplitude-modulated pump signal is especially effective with a frequency that lies above the Brillouin bandwidth, usually between 20 MHz and 500 MHz. Higher frequencies can also be employed given low-dispersion fibers.
In order to keep the pump power of each and every pump laser low, a plurality of amplitude-modulated pump signals should be supplied that are optimally uniformly distributed in terms of wavelength and, for example, lie at the edges and in the gaps of the transmission band. The frequency of the phase of all amplitude-modulated pump signals should be the same so that the effects add up. Due to the low pump powers and the skillful selection of the wavelength, signal disturbances due to stimulated Raman scatter are largely prevented. The Raman scatter potentially caused by amplitude-modulated pump signals can be utilized, as warranted, for improving the transmission properties, such as a correction of the tilt.
A broadband or directly modulated, chirping laser or an incoherent light source can be employed as a source of the amplitude-modulated pump signal.
A broadband pump signal can also cover the payload channels when the required noise ratio is not downwardly exceeded as a result thereof, for example given wave division multiplex systems with a channel data rate up to 2.5 Gbit/s.
Other advantages and features of the invention will be readily apparent from the following description of the preferred embodiments, the drawings and claims.